Generally, in joints having multiple fasteners it is desirable that the fasteners fit net or be at an interference fit within the fastener holes since this tends to better distribute the load among adjacent fasteners. This is particularly true when the materials to be joined have very little ductility, such as fiberglass/epoxy and graphite/epoxy laminates, for such materials unlike aluminum and other structural materials used in the aircraft industry, do not help distribute the load by plastically deforming to relieve the stresses. Such yielding helps to distribute the load more evenly among the fasteners in metallic structures and as a consequence reduces the probability of premature failure.
In joints which are subjected to tension-compression fatigue loads, if a clearance fit is used, the bolt shank will pound the composite within the hole at each load reversal resulting in elongated holes. This leads to possible joint failure. This pounding and hole elongation does not occur with interference fit fasteners.
Although as noted, interference fit is desirable, the present practice of obtaining interference fit bolted joints in laminates leads to some difficulties, because laminates tend to delaminate when fasteners are installed in net or interference fit. This delamination is caused by the pushing action of the cylindrical surface of the fastener as the fastener is forced in the hole. This forcing tends to delaminate the last plies as the fastener emerges on the far side of the fastener hole. Special means must be provided to prevent this, usually taking the form of a metal backup plate. However, there are many instances where limited access to the back side of a joint does not permit the use of backup plates or makes their use extremely inconvenient.
Joints incorporating laminates and especially graphite/epoxy laminates must be put together carefully using the right combination of materials since contact between incompatible materials will result in the formation of a galvanically active joint which may lead to failure. Therefore, there has been a need to provide a joint, especially for graphite/epoxy structures which is galvanically inactive and results in a long life without deterioration of either the static or fatigue strength of the joint. The joint fabrication should be amenable to a mass production environment and should be relatively inexpensive both in labor time and consumables.